Selected heavy ions have been shown to be potent inducers of mutations in human TK6 lymphoblasts at two distinct genetic loci: thymidine kinase (tk) and hypoxanthine phosphoribosyltransferase (hgprt). The kinetics for mutation induction appear to depend on the genetic context of the particular locus in addition to LET. A systematic investigation of the dependence of mutation induction on particle charge, velocity and fluence is outlined for both the hemizygous hgprt locus and the heterozygous tk locus. Specifically, the hypothesis that considerations such as gene dosage and linkage to essential genes influence not only the kinetics of induction but also the LET vs RBE response for a given genetic locus will be tested. Secondly, we will determine whether the mutation rate may saturate more readily for a hemizygous locus with respect to particle fluence than for a heterozygous locus. This may be a result of the propensity with which accelerated heavy ions induce large scale genetic changes, including extensive deletions, such that the magnitude of those alterations resulting in viable mutants is limited by genetic linkage. This assumption will be tested by examining the spectrum of DNA structural alterations in clonally-derived populations of tk-/- or hgprt- mutants arising in cultures exposed to a single particle beam. Intragenic and multilocus change will be examined to determine the magnitude and polarity of structural alterations. The effect of particle fluence on the spectrum of DNA structural changes for each locus will be examined. The dependence of the mutation spectrum for a given locus on the absorbed dose to the nucleus will also be explored. Finally, the possibility that cryptic lethal alleles linked to the active tk or hgprt alleles of TK6 cells influence the spectrum of recoverable mutants will be examined by introducing an additional copy of chromosome 17 containing the gene for neomycin resistance into mutant TK6 subclones harboring large deletions of active tk or hgprt sequences. Neo+ revertants to tk+, or revertants to hgprt+ will be remutagenized with a heavy-ion beam. The spectrum of DNA structural alterations in neo+/tk- or newly made hgprt- lines will be compared with those obtained for tk- or hgprt- mutants of wild-type TK6 cells after exposure to the same heavy-ion beam. These studies will contribute to our understanding of the influence of genetic constraints on possible mechanisms of mutagenesis in human cell by densely ionizing radiations.